The principal function of automotive steering wheels is, of course, steering the car. In addition, steering wheels which are kept in contact with the driver's hands while the car is running must satisfy several other requirements, among which the following are important; (1) good appearance, (2) good touch to the hands (non-stickiness plus "soft touch" which is gaining increasing acceptance by drivers), (3) freedom from deteriorations due to sunlight such as discoloration, cracking and bleeding of additives, and (4) resistance to wear due to constant friction with the hands.
Soft vinyl chloride resins are commonly used as the material for steering wheels because of their relatively low cost and good weathering properties. However, it is very difficult to produce injection moldings having satisfactory wear resistance and a uniform matted appearance. The reasons are as follows: (1) the most effective way to provide a vinyl chloride resin with an improved wear resistance without increasing its hardness is to increase the average polymerization degree as determined by the solution viscosity method (hereunder abbreviated as P) of the resin, but if P is increased, the corresponding decrease in the fluidity of the resin will most likely cause uneven luster in the surface of the injection molded article; (2) if an inorganic material such as calcium carbonate or talc is added in an attempt at providing a matted appearance, the wear resistance of the resin is reduced and uniformity in luster is not obtained while only an incomplete matting effect is achieved; (3) a mold having an embossed pattern is highly effective for the purpose of providing a matted appearance but no appreciable improvement in matte uniformity is obtained, and in addition, this method does not ensure consistent product quality in mass production since the embossed pattern wears as the number of molding cycles is increased. For these reasons, coating is necessary for imparting uniform luster to conventional steering wheels made of soft PVC (hardness.ltoreq.70 Hs, type A according to JIS K 6301). However, a relatively good uniformity in luster can only be obtained at the expense of wear resistance. Furthermore, coated PVC steering wheels are not only expensive but also prone to suffer peeling problems under hostile conditions.
Crosslinked vinyl chloride polymers or compositions made of crosslinked vinyl chloride polymers and non-crosslinked polymers could be used for providing matting effects as shown in several prior art references such as Japanese Pat. Application (OPI) Nos. 117550/1979, 78044/1980, 78055/1980, 5843/1981, 92024/1981, 195710/1982, 195711/1982, 195712/1982, and 195713/1982 (the term "OPI" as used herein mean a "published unexamined Japanese Patent Application"). The polymers or compositions shown in these references are effective for providing satisfactory matting effects, but they are entirely ineffective for the purpose of providing high matte uniformity, and no saleable products are obtainable without coatings.
The reason for these difficulties is that conventional quality evaluation techniques are directed primarily to products made by rolling, calendering and extrusion molding, and in a few cases, to injection moldings of very simple configurations. Therefore, such techniques are ineffective for evaluating the uniformity in luster of injection molded steering wheels having a complex shape. In order to achieve a uniform luster in the products shaped by rolling, calendering or extrusion molding techniques, primary care should be taken in the width direction although controlling the uniformity in the length direction is relatively easy. On the other hand, the entire surface of the article being shaped by injection molding must be controlled for providing the desired uniformity in luster, and this is why considerable difficulty is involved in achieving high uniformity in luster by injection molding techniques.
With injection molding, providing a matted appearance for the entire surface of complex shapes such as steering wheels involves a much greater difficulty than in the case of simple shapes such as flat plates, rods and trays. In addition to the complexity of its shape, the steering wheel has an iron insert placed not only on the periphery of the ring portion but also in the spokes. Because of these inserts, a resin stream bumps violently against various parts of the mold to experience a sudden change in its flowing direction. Therefore, the steering wheel prepared by injection molding has an inherent tendency to produce a glossy surface. The molten resin should have an optimum viscosity for achieving a high uniformity in its gloss. A resin stream having a higher viscosity will deform or displace the inserts in the steering wheel. On the other hand, the higher the molecular weight of the resin, the more wear resistant the resulting steering wheel. Since high wear resistance is required of the steering wheel, this tradeoff between uniformity in luster and wear resistance introduces considerable difficulty in designing steering wheels.